Ethylenediamine-Assisted Ligand Exchange and Phase Transfer of Oleophilic Quantum Dots: Stripping of Original Ligands and Preservation of Photoluminescence

We demonstrated a facile and general method to transfer oleophilic quantum dots (QDs) to the aqueous phase with a high transfer efficiency (∼100%) and essentially retaining the photoluminescence quantum yield (>80%). The transfer is based on a ligand-exchange process mediated by ethylenediamine (EDA). The addition of EDA prior to strong bifunctional ligands (i.e., mercaptocarboxylic acids) to oleophilic QDs was proved to be the critical step. The roles of EDA in this process are the following: (1) reacts with and strips off original ligands; (2) reversibly adsorbs on the surface of stripped QDs in the organic phase and transfers QDs to the aqueous phase; and (3) spontaneously desorbs from the QD surface in the aqueous phase and leaves reactive surface sites to strong ligands. These three roles of EDA work convergently to facilitate efficient transfer of QDs while essentially preserving surface properties of the solid component of QDs. In addition, the EDA-capped QDs were found to be substantially reactive toward incoming strong ligands, due to the complete opening of surface reactive sites to the environment. This method is promising to obtain high-quality QDs in the aqueous phase for subsequent surface functionalization (DNA, protein, peptides, etc.) for various biomedical applications.